This invention relates to Ni-Zn batteries characterized by longevity at high levels of retained capacity and power, and particularly to full-sized batteries suitable for electric vehicle and SLI applications. Such Ni-Zn batteries will comprise a plurality of nickel electrode plates (i.e., NiOOH in charged state) closely interleafed with a plurality of zinc electrode plates (i.e., zinc in the charged state). More particularly, this invention relates to zinc electrodes for such batteries.
Zinc electrodes have heretofore typically been made by: homogeneously mixing zinc oxide powder with such customary binder materials as polystyrene, polytetrafluoroethylene, polyvinylidene fluoride, methylcellulose, carboxymethylcellulose, polyvinylalcohol or polyethylene oxide; applying the mix to a conducting/supporting gridwork; and pressing the mix to hold the powders together. During formation (i.e., initial charge), the zinc oxide transforms to zinc. In some cases, such electrodes have demonstrated signs of segregation of the zinc, zinc oxide and binder--especially toward the ends of their useful lives. In other cases (i.e. with the water-soluble/KOH-insoluble cellulosic type binders), such electrodes have demonstrated signs of binder migration and the formation of films of the binder on the surface of the electrodes.
Such electrodes have heretofore generally demonstrated: relatively poor zinc utilization efficiency; relatively poor capacity retention and power retention upon cycling (i.e., charging and discharging)--often falling below a useful level, for many applications, after too few cycles; and relatively short useful lives. To offset these shortcomings, workers in the field have typically provided nickel-zinc batteries with approximately three times more zinc electrode capacity than nickel electrode capacity [e.g., 300 AH of zinc oxide per 100 AH Ni(OH).sub.2 in the discharged state].
Over the years, a number of additions to the zinc electrode have been proposed. Small additions of lead (i.e., as Pb, PbO or Pb.sub.3 O.sub.4) or copper powder have been suggested to promote better utilization of the zinc and hence improve the capacity of the electrode. It has likewise been proposed to add H.sub.2 -gas suppressants to raise the hydrogen overvoltage and thereby reduce H.sub.2 gassing while charging the battery. Among the H.sub.2 suppressants suggested are Tl.sub.2 O.sub.3, PbO, CdO, and mixtures thereof with each other and with SnO.sub.2, In(OH).sub.3 and Ga.sub.2 O.sub.3. These materials have also been said to be effective in reducing zinc electrode "shape change".
"Shape change" is a phenomenon which results from an uneven deposition of the zinc on the electrode on succeeding charge and discharge cycles. More zinc tends to accumulate in the center of the electrode where it becomes dense and nonreactive while the edges of the electrode become depleted of zinc. The net result is a deformed electrode having a very small effective surface area available for reaction. While the "shape change" phenomenon is not fully understood it is seen to commonly occur in cells having high zinc mobility and on plates having uneven current distributions thereacross. This condition is amplified as the surface area of the electrode increases and has been a particular problem in full-sized electric vehicle and SLI batteries where the zinc plates must often exceed 25 in.sup.2 in area in order to provide adequate energy and power.
Calcium compounds (i.e., calcium oxide and/or calcium hydroxide) have been added to zinc electrodes to promote the formation of calcium zincate (i.e., CaZn.sub.2 O.sub.8 H.sub.10) during cell discharge. In such calcium-rich electrodes, the calcium reacts with the zinc oxide formed on cell discharge (i.e. said by some to be "potassium zincate") to form relatively insoluble calciun zincate (hereafter Ca-zincate). This increases cell cycle life by reducing zinc mobility and reducing the formation of zinc dendrites which can bridge the interelectrode gap and short out the cell. By way of contrast, in calcium-free cells the zinc oxide reaction product is quite soluble in the KOH electrolyte hence creating loss of zinc from the electrode and high zinc mobility throughout the cell. Such zinc mobility is believed to contribute to both zinc dendrite formation and zinc electrode "shape change".
Though the aforesaid advantages of adding calcium to zinc electrodes has been demonstrated in laboratory test cells, surprisingly it has not been reported to have been adopted for use in full-sized batteries. The reason for this is not known. Perhaps it is due to the tendency of Ca-zincate toward cementation, as has been reported by some. Perhaps it is due to the difficulty of duplicating, in full-sized batteries, the results observed in the laboratory cells.
I have found that irrigation of Ca-rich zinc electrodes with dilute electrolyte (i.e., concentrations below about 30% by weight KOH) yields long-lived electrodes with high levels of retained capacity and retained power in full-sized batteries and all with no evidence of Ca-zincate cementation or reduced Ca-zincate effectiveness. The concentration of the electrolyte within the zinc electrode significantly affects the effectiveness of the calcium in consuming the zinc as Ca-zincate. When the electrolyte concentration exceeds a certain level (hereafter Ca-zincate dissociation concentration), Ca-zincate will neither form in substantial quantities nor, if earlier formed, persist in significant amounts. This concentration varies inversely with respect to temperature. That is to say, as the temperature increases the Ca-zincate dissociation concentration decreases over the normal operating temperatures of electric vehicle and SLI batteries. Hence Ni-Zn batteries, and particularly zinc electrodes, must be designed to insure that the concentrations of the electrolyte within the zinc electrode does not exceed the Ca-zincate dissociation concentration. Achievement of this goal is complicated by the fact that the Ca-zincate formation reaction consumes significantly more water (H.sub.2 O) than does the potassium zincate formation reaction occuring in Ca-free electrodes. As a result, during discharge (i.e., when the Ca-zincate is being formed), the concentration of the electrolyte contained within the zinc electrode (hereafter the electrolyte permeate) can quickly rise above the Ca-zincate dissociation concentration even though the ambient electrolyte between and around the electrodes remains well below the dissociation concentration. The higher the discharge rate the faster the water is consumed and the greater the need for adequate irrigation.
I have concluded that the benefits of calcium in zinc electrodes can only be achieved by proper management of the concentration of the electrolyte permeate and more specifically by keeping that concentration below the Ca-zincate dissociation concentration over the operating temperature range of the battery. The problem of electrolyte concentration management is particularly acute in full-sized, multiplate, commercial batteries which must typically operate in an electrolyte-starved environment (i.e., low electrolyte-to-active material ratio) and over a wide temperature range.
It is the principal object of the present invention to provide a long-lived, Ni-Zn battery having improved zinc utilization and excellent retained capacity upon repeated cycling and which is characterized by a zinc electrode whose active material contains a Ca-zincate former and is pervaded with porous, wick-like, absorbent, hydrophilic fibers for so irrigating the active material with dilute electrolyte as to permit the formation and persistence of Ca-zincate therein during discharge of the battery and to reduce any tendency of the Ca-zincate toward cementation.
It is a further object of the present invention to provide a long-lived, Ni-Zn battery having improved zinc utilization and excellent retained capacity and power upon repeated cycling and which is characterized by a zinc electrode which is essentially free of customary binder materials and whose active material includes a Ca-zincate former and is pervaded with an entanglement of stable, reinforcing, porous, wick-like, absorbent, hydrophilic fibers for so irrigating the active material with dilute electrolyte as to permit the formation and persistance of Ca-zincate therein during discharge of the battery and to reduce any tendency of the Ca-zincate toward cementation.
It is another object of the present invention to provide a long-lived, Ni-Zn battery having improved zinc utilization and excellent retained capacity and power upon repeated cycling and which is characterized by a zinc electrode whose active material includes a Ca-zincate former and a hydrogen gas suppressant, and is pervaded with porous, wick-like, absorbent, hydrophilic fibers for so irrigating the active material with dilute electrolyte as to permit the formation and persistence of Ca-zincate therein during discharge of the battery and to reduce any tendency of the Ca-zincate toward cementation.